The invention relates to method of retrofitting a head assembly of a reactor pressure vessel (RPV) in a pressurized water nuclear reactor (PWR) for facilitating the removal and reinstallation of the RPV""s closure head during a refueling operation and, more particularly, to a method for retrofitting a RPV in a pressurized water nuclear reactor having ice condensers for absorbing large amounts of heat in the event of a severe accident.
In commercial PWRs, RPVs have control rods for controlling the nuclear reaction in fuel assemblies located in their core regions. The control rods are vertically operated by assemblies known as control rod drive mechanisms (CRDMs). The CRDMs are vertically supported on removable closure heads bolted to the RPVs, laterally supported by seismic support platforms and vertically restrained by overhead missile shields. In addition to supporting the CRDMs, the closure heads mechanically support complex ventilation systems located above the closure heads for providing substantial, continuous flow of ambient containment air to cool the CRDM electromagnetic coils. See, in this regard, FIG. 1 of U.S. Pat. No. 4,678,623, which illustrates a head arrangement found in many commercial facilities.
During refueling operations, the RPV closure heads, CRDMs and their supporting subsystems and other devices located over the closure heads must be disassembled, lifted and removed so that the RPV closure heads can be removed and spent fuel assemblies in the core regions of the RPVs can be rearranged or replaced with fresh fuel assemblies. To reduce the time required to remove and reinstall RPV closure heads in order to refuel nuclear reactors, integrated head assembly designs were developed in the 1980s as backfits for the type of design discussed above. U.S. Pat. No. 4,678,623 shows a backfit integrated head assembly wherein elbow ducts 22 extending from a lower manifold 20 to an upper manifold 24 located over a missile shield 34 (as shown in FIG. 1) were replaced with a duct arrangement 136, 138 and 140 (as shown in FIG. 2). More recently, the integrated head assembly designs have been simplified to further reduce the time required to remove and reinstall the RPV closure heads and thereby to reduce radiation exposure by operating personnel. See, in this regard, U.S. Pat. No. 5,742,652 and 5,930,321. The disclosures U.S. Pat. No. 4,678,623; 5,742,652 and 5,930,321 are incorporated by reference for their disclosures of the structures and functions of integrated head assemblies and simplified head assemblies.
For several reasons, the integrated head assembly designs and the later simplified head assembly designs have not been employed in PWRs having ice condensers in compartments for absorbing large amounts of heat in the event of a severe accident. See, in this regard, U.S. Pat. No. 4,238,289, which illustrates in FIG. 1a PWR containment building 10 containing ice condenser compartments 18 located above an operating deck 24 and a RPV 26 located below the operating deck 24 in a tight RPV compartment defined in part by primary shields 46 and a large heavy concrete missile shield 50. First, the concrete missile shields of ice condenser plants provide pressure boundaries for loss of coolant events. Second, the clearances between the PWR plants the missile shields and the seismic support platforms for laterally supporting the CRDMs are so limited that there is not enough space for the integrated head designs and simplified head designs that have been developed. Third, compartmentalization in the containment buildings in which the PRVs are located is such that the heat removed from the CRDM cooling air must be removed before the air can be discharged back into the general atmosphere in the containment buildings. In addition, the integrated head designs and the simplified head designs do not provide cooling prior to discharge of the cooling air into the general atmosphere in the containment building.
The nuclear industry has developed a modified head assembly design for RPVs in ice condenser plants for reducing refueling times and radiation exposures. For the reasons stated above, the modified design doesnot incorporate a missile shield or a permanently attached lift rig. The modified design includes a RPV closure head, CRDMs, seismic support plate and a CRDM cooling shroud. The modified design also includes connections with the outlets of cooling air ductwork located in the RPV compartments between the seismic support plates and the RPVs for directing cooling air from the general atmosphere within the containment building to the CRDMs. In the course of refueling operations, scaffolding must be erected over the RPV compartment after the missile shield has been removed for connecting and disconnecting the cooling air ductwork before the head assembly can be removed.
Although the modified head assembly has reduced refueling times and radiation exposures, the nuclear industry desires to further reduce refueling time and radiation exposures with improved safety.
It is an object of the present invention to provide a method for backfitting the PRV head assemblies in ice condenser plants in order to permit their removal and reinstallation during a refueling operations in a shorter period of time and with reduced radiation exposure. It is a further object to provide a design that can be backfit into the limited space of commercial ice condenser plants.
With these objects in view, the present invention resides in a method for retrofitting a RPV head assembly in an existing ice condenser type PWR generally having a containment building containing the RPV, a missile shield superposed over the RPV head assembly and a ventilation system for directing cooling air from the atmosphere within the containment building toward the head assembly during power operations.
A head assembly to be retrofitted generally includes a RPV head, a plurality of CRDMs extending upwardly from the RPV head, a seismic support platform above the RPV head laterally supporting the CRDMs, and a CRDM cooling shroud surrounding a portion of the CRDMs and extending upwardly from the RPV head to a terminal end spaced from the seismic support platform. The ventilation system generally includes ductwork extending from one or more ventilation fans and heat exchangers remote from the RPV to outlets located adjacent the head assembly between the RPV closure head and the seismic support platform. The ductwork outlets may be connected to radiation shields or other structures extending upwardly of the RPV closure heads.
The practice of the method of retrofitting the RPV head assembly generally includes the steps of: removing at least a portion of the ventilation ductwork extending below the seismic support platform; extending the CRDM cooling shroud to the seismic support platform; mounting a plenum on the seismic support platform and in air flow communication with interior portion of the extended CRDM cooling shroud and under the missile shield; and connecting one end of a removable spool piece to the plenum and a second end of the removable spool piece to the ductwork adjacent to the seismic support platform. Advantageously, in the course of later refueling operations, the RPV integrated head assembly can be disconnected from and reconnected to the ductwork of the ventilation system without interference with a detachable lift device and without the need for scaffolding around the RPV integrated head assembly.